Conventionally, as a thermionic emission filament of this type, there has been known that an electron emitting layer is formed by coating a surface of a core member made of iridium with an electron emitting substance made of yttrium oxide. As disclosed in Patent Literature 1 and Non-Patent Literature 1, the electron emitting layer is conventionally coated on the surface of the core member by an electrophoresis method.
However, in the case where the thermionic emission filament is used, for example, in a mass spectrometer such as residual gas analyzer for analyzing residual gas in a semiconductor process chamber, corrosive gasses such as fluorine gas used for cleaning inside the chamber may be possibly contained in the gas to be analyzed. In such a case, since the electron emitting layer formed by an electrophoresis method has no denseness, the corrosive gasses may penetrate through clearances of the electron emitting layer and reach the core member. Therefore, the core member is corroded and the thermionic emission filament is liable to be broken earlier than an expected lifetime.
Therefore, in the case where the thickness of the electron emitting layer is increased in order to protect the core member from corrosion, since the thermionic emission filament is still liable to be broken earlier than an expected lifetime due to thermal stress.
Further, in order to extend the life of the thermionic emission filament, although it appears to be considered that, after the residual gas of a low concentration in the semiconductor process chamber is further diluted, the gas is analyzed by a mass spectrometer, there arises a problem that the analyzing accuracy is deteriorated.